There is ongoing interest in expanding and improving the measurement of semiconductor wafers. A number of optical metrology tools have been developed for non-destructively evaluating the characteristics of thin films formed on semiconductors during the fabrication process. More recently, optical metrology systems have been proposed for analyzing the geometry of small periodic structures (critical dimensions) on semiconductors.
Typical optical tools include reflectometry (both single wavelength and spectroscopic) and ellipsometry (again, both single wavelength and spectroscopic). In some metrology tools, these various techniques are combined. See for example U.S. Pat. Nos. 6,278,519 and 5,608,526, the disclosures of which are incorporated herein by reference.
Other metrology tools have been developed which rely on measurements at multiple angles of incidence (both single wavelength and spectroscopic). One class of such systems have been commercialized by the Assignee herein are capable of deriving information about multiple angles of incidence simultaneously. In these systems, a strong lens (high numerical aperture) is used to focus a probe beam of light onto the sample in a manner to create a spread of angles of incidence. An array detector is used to measure the reflected rays of the probe beam as a function of the position within the probe beam. The position of the rays within the probe beam corresponds to specific angles of incidence on the sample. These systems are disclosed in U.S. Pat. Nos. 4,999,014 and 5,042,951, incorporated herein by reference. U.S. Pat. No. 4,999,014 relates to reflectometry while U.S. Pat. No. 5,042,951 relates to ellipsometry. (See also, U.S. Pat. No. 5,166,752, also incorporated herein by reference.)
In a variant on this system, U.S. Pat. No. 5,181,080 (incorporated by reference), discloses a system in which a quad-cell detector (FIG. 1) is used to measure the reflected probe beam. Each quadrant 1-4 of the detector measures an integration of all of the angles of incidence falling on the sample. By subtracting the sums of opposite quadrants, ellipsometric information can be obtained. As described in the latter patent, the information derived from the analysis corresponds to the ellipsometric parameter delta δ which is very sensitive to the thickness of very thin films on a sample.
The concepts of the latter patents were expanded to provide spectroscopic measurements as described in U.S. Pat. No. 5,412,473, also incorporated herein by reference. In this patent, the system was modified to include a white light source. In one approach, a color filter wheel was used to sequentially obtain multiple wavelength information. In another approach, a filter in the form of a rectangular aperture was used to select a portion of the reflected beam. This portion was then angularly dispersed onto an array with each row providing different wavelength information and each column containing the various angle of incidence information.
U.S. Pat. No. 5,596,411, also incorporated by reference, disclosed a preferred approach for obtaining spectroscopic information for an integrated multiple angle of incidence system of the type described in U.S. Pat. No. 5,181,080 discussed above. In this approach, a filter was provided that transmitted light along one axis and blocked light along an orthogonal axis. The transmitted light was angularly dispersed and measured to provide spectroscopic information along one axis of the probe beam. The filter was then rotated by ninety degrees to obtain measurements along the remaining axis. Various modifications of this approach were discussed, including splitting the beam and using two identical filters disposed orthogonal to each other to obtain both measurements simultaneously. (See also “Characterization of titanium nitride (TiN) films on various substrates using spectrophotometry, beam profile reflectometry, beam profile ellipsometry and spectroscopic beam profile ellipsometry,” Leng, et al., Thin Solid Films, Volume 313-314, 1998, pages 309 to 313.)
The integrated multiple angle ellipsometric measurement system described in U.S. Pat. No. 5,181,080, cited above has been successfully commercialized and is incorporated into the Opti-Product sold by the Assignee herein. The technology is marketed under the trademark Beam Profile Ellipsometry. (See U.S. Pat. No. 6,278,519 cited above.) As described in the '080 patent, the four segments of the quad cell detector can be summed to provide information about the total reflected power of the probe beam. In addition, the sum of the output of the quadrants along one axis can be subtracted from the sum of the outputs of the remaining two quadrants to provide a result which corresponds to the ellipsometric parameter δ.
This arrangement provides valuable information that can be used to determine the thickness of thin films. However, the limited information from this type of detection cannot typically be used to derive both of the ellipsometric parameters, Ψ and δ. U.S. Pat. No. 5,586,411, discloses that it would be possible to derive such information if one of polarizers were rotated and multiple measurements taken. As noted therein at column 12, line 48, if enough measurements are taken, a Fourier analysis can be performed on the data allowing the parameters of Ψ and δ to be extracted.
When designing commercial inspection systems, it is often desirable to minimize the number of moving parts. For example, moving parts often create particulates that can contaminate the wafer. To the extent parts must be moved, the motion systems must have high precision. Further, movements of parts that are specifically designed to modify optical properties, such as retarders or polarizers can effect how the system transmits and detects light.
Therefore, it is an object of the present invention to enhance the operation of an integrated, simultaneous multiple angle ellipsometric system without the drawbacks of the prior approaches. In particular, the subject invention is intended to permit the derivation of additional ellipsometric information, including both δ and Ψ. In one class of embodiments, this additional information is derived in a system with an improved detector arrangement without the need for moving parts. In another class of embodiments, the rotating element is limited to the detector which does not effect the polarization or retardation of the light.